<html><body><div style="color:#000; background-color:#fff; font-family:times new roman, new york, times, serif;font-size:12pt"><div><span>Hi Jim and Pavan,</span></div><div style="color: rgb(0, 0, 0); font-size: 16px; font-family: times new roman,new york,times,serif; background-color: transparent; font-style: normal;"><br><span></span></div><div style="color: rgb(0, 0, 0); font-size: 16px; font-family: times new roman,new york,times,serif; background-color: transparent; font-style: normal;"><span>Thank you for your replies. I meant the time I'd get by calling MPI_Wtime() .</span><span> Section 7.1.1.4 (Clock Synchronization) on http://www.mpi-forum.org/docs/mpi-11-html/node143.html states "the expectation is </span>that the variation in time, as measured by calls
to MPI_WTIME, will be less then one half the round-trip
time for an MPI message of length zero."<br></div><div style="color: rgb(0, 0, 0); font-size: 16px; font-family: times new roman,new york,times,serif; background-color: transparent; font-style: normal;"><br></div><div style="color: rgb(0, 0, 0); font-size: 16px; font-family: times new roman,new york,times,serif; background-color: transparent; font-style: normal;">So I guess every time I call MPI_Init(), the clocks (per process) are synchronized. My original question was what algorithm is used to sync that time across multiple machines. However if the variation is a few 10s of ms I should probably be fine. I'd been in trouble if it was 100s of ms. I've got only 3-4 machines, so hopefully it shouldn't be too bad.<br><span></span><span></span><span></span></div><div style="color: rgb(0, 0, 0); font-size: 16px; font-family: times new roman,new york,times,serif; background-color: transparent; font-style: normal;"><br></div><div style="color: rgb(0, 0, 0);
font-size: 16px; font-family: times new roman,new york,times,serif; background-color: transparent; font-style: normal;">Thanks,</div><div style="color: rgb(0, 0, 0); font-size: 16px; font-family: times new roman,new york,times,serif; background-color: transparent; font-style: normal;">Ravi<br><span></span><span></span></div><div><br></div><div style="color: rgb(0, 0, 0); font-size: 16px; font-family: times new roman,new york,times,serif; background-color: transparent; font-style: normal;">[please keep mpich-discuss cc'ed].<br><br>There is no real communication during synchronization. Zero-byte <br>messages are exchanged. The algorithm used is logarithmic with respect <br>to the number of processes.<br><br> -- Pavan<br><br>On 04/02/2011 02:52 AM, <span class="yshortcuts" id="lw_1301800860_0">Ravi Prakash</span> wrote:<br>> Thanks Pavan! :) Do you know if synchronization is done by splitting the<br>> communication time in half or somesuch
algorithm?<br>><br>> Cheers<br>> Ravi<br>></div><div style="font-family: times new roman,new york,times,serif; font-size: 12pt;"><div style="font-family: times new roman,new york,times,serif; font-size: 12pt;"><font face="Arial" size="2"><hr size="1"><b><span style="font-weight: bold;">From:</span></b> James Dinan <dinan@mcs.anl.gov><br><b><span style="font-weight: bold;">To:</span></b> mpich-discuss@mcs.anl.gov<br><b><span style="font-weight: bold;">Sent:</span></b> Saturday, April 2, 2011 8:56 AM<br><b><span style="font-weight: bold;">Subject:</span></b> Re: [mpich-discuss] Clock synchronization<br></font><br>
Hi Ravi?<br><br>Do you mean synchronization of the time of day on the machines? If so, <br>MPI doesn't synchronize the clock on each machine. If you want to do <br>this, you need a network time daemon or whatever equivalent there is for <br>windows. This is outside of the scope of MPI, but someone else may be <br>able to suggest what you need.<br><br>Best,<br> ~Jim.<br><br>On 4/1/11 11:28 PM, Ravi Prakash wrote:<br>> Hi,<br>><br>> I'm a n00b and I'm wondering if there is any documentation on what clock<br>> synchronization algorithm MPICH2 uses. I need to trigger events at the<br>> same time across multiple machines. What accuracy am I looking at?<br>><br>> My machines are Windows Vista boxes with Dual dual-core Xeons connected<br>> by an ethernet 100Mbps LAN. Please let me know if there are any more<br>> relevant factors needed for an approximate figure. Is it a few ms / 10's<br>> of ms / 100's of ms
/ god forbid seconds? :)<br>><br>> Cheers<br>> Ravi<br>><br>><br>><br>> _______________________________________________<br>> mpich-discuss mailing list<br>> <a ymailto="mailto:mpich-discuss@mcs.anl.gov" href="mailto:mpich-discuss@mcs.anl.gov">mpich-discuss@mcs.anl.gov</a><br>> <a href="https://lists.mcs.anl.gov/mailman/listinfo/mpich-discuss" target="_blank">https://lists.mcs.anl.gov/mailman/listinfo/mpich-discuss</a><br><br>_______________________________________________<br>mpich-discuss mailing list<br><a ymailto="mailto:mpich-discuss@mcs.anl.gov" href="mailto:mpich-discuss@mcs.anl.gov">mpich-discuss@mcs.anl.gov</a><br><a href="https://lists.mcs.anl.gov/mailman/listinfo/mpich-discuss" target="_blank">https://lists.mcs.anl.gov/mailman/listinfo/mpich-discuss</a><br><br><br></div></div></div></body></html>